model a shape something like a raspberry
Take a look at Dupliverts, that allows the duplication of a base object at the location of the vertices of a mesh. In other words, when using DupliVerts on a mesh, an instance of the base object is placed on every vertex of the mesh.

Or try the Tissue add-on by Alessandro Zomparelli

It allows you to copy a selected object (Component) on the faces of the active object (Generator), adapting its bounding box to the shape of quad-faces.



Thread on Blenderartists.org with more examples here